1. Field of the Invention
The invention relates to electrostatic chucks and, more particularly, the invention relates to in-situ measurement and control of a potential on the surface of an electrostatic chuck.
2. Description of the Background Art
Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor wafer process chamber. In semiconductor wafer processing equipment, electrostatic chucks are used for clamping wafers to a pedestal during processing. These chucks find use in etching, chemical vapor deposition (CVD), and physical vapor deposition (PVD) and other applications.
Electrostatic chucks secure a workpiece by creating an electrostatic attractive force between the workpiece and the chuck. A voltage is applied to one or more electrodes that are embedded in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The opposite charges pull the workpiece against the chuck, thereby retaining the workpiece. For example, in a "monopolar" electrostatic chuck, voltage is applied to a single conductive chuck electrode that is embedded within a dielectric or semiconductive chuck body. The magnitude of the chucking voltage is relative to some ground reference. When the voltage is applied, the workpiece is referred back to the same ground reference as the voltage source by a conductive connection to the workpiece. Electrostatic force is established between the workpiece being clamped and the electrostatic chuck. A "bipolar" electrostatic chuck generally contains two electrodes embedded within a unitary dielectric or semiconductive chuck body. When a chucking voltage is applied between the two electrodes, a small current flows between the electrodes and through the workpiece such that oppositely polarized charges respectively accumulate on the backside of the wafer and on the surface of the chuck body. These charges establish an electrostatic force, between the chuck and the workpieces via the Johnsen-Rahbek effect.
In either type of electrostatic chuck, a surface potential appears on the dielectric above the electrodes when a voltage is applied to the electrodes. The surface potential is directly proportional to the chucking force. For an ideal dielectric, the surface potential is equal in magnitude to the voltage on the underlying chuck electrode. Charging effects, polarization and other material specific phenomena can cause the surface potential on top of the dielectric to be different from the voltage on the underlying electrode. As a result, the performance (i.e., chucking force) of the chuck is affected. For example, some chucks exhibit a degradation of the surface potential over time after a chucking voltage has been applied to the chuck electrodes. This is believed to be due to the existence of low mobility charge carriers in the bulk dielectric material of the chuck. Other chucks exhibit a transient repelling force when the chucking voltage is turned off. This is due to repulsion of residual charges on the wafer backside by charges of like polarity induced on the chuck electrodes by a transient overpotential on the chuck electrodes when the chuck is turned off. The chucking performance varies in an unpredictable fashion from chuck to chuck. Furthermore, the behavior of a given chuck varies in an unpredictable fashion over time.
Active control of the surface potential requires that surface potential measurement be made in situ during wafer processing. Furthermore, reliable measurement and active control of the surface potential require that the surface potential be measured at a fixed location on the chuck surface in order to compare measurements of the surface potential taken over time. In the prior art, electrostatic probes and meters have been used to measure the surface potential of electrostatic chucks. Unfortunately, the probes used for these measurements are not suitable for the often harsh environment that exists inside semiconductor wafer processing chambers when the chamber is in use. Consequently, such probe measurements are not performed while the chamber is operating and therefore must be made in air, when the chamber is open, or when the chuck is removed from the chamber.
Therefore, a need exists in the art for an apparatus and method for reliably measuring and actively controlling electrostatic chuck surface potential during wafer processing.